Method of second harmonic control



Sept. 3, 1963 J. M. DREES 3,102,597

METHOD OF SECOND HARMONIC CONTROL Filed Aug. 14, 1961 3 Sheets-Sheet 2 INVENTOR ATTORNEY p 3, 1963 J. M. DREES 3,102,597

METHOD OF SECOND HARMONIC CONTROL Filed Aug. 14, 1961 3 Sheets-Sheet 5 F/G 9 INVENTOR J4/V M 0 9555 BY MW N ATTORNEY United States Patent 3,102,597 METHOD OF SECOND HARMONIC CONTROL Jan M. Drees, Dallas, Tex., assignor to Bell Aerospace, Wheatfield, N.Y., a corporation of Delaware Filed Aug. 14, 1961, Ser. No. 131,154 7 Claims. (Cl. 170-16025) The present invention relates to rotary wing aircraft, particularly a method for simply developing second harmonic control of variable phase and amplitude.

Cyclic pitch control by conventional swashplate applies a first harmonic feathering control to the helicopter blades as they rotate, each blade going through a complete cycle of change in pitch once per revolution. In forward flight or while hovering in a breeze the blade advancing in the wind has a relative air speed greater than that of the blade retreating away from the wind, tending to create greater lift on the advancing blade than on the retreating blade. The swashplate-induced first harmonic feathering control compensates forthis difference in lift by increasing the angle of incidence on the retreating blade and decreasing the angle of incidence on the advancing blade, so that the lift of the advancing blade is approximately equal to the lift of the retreating blade. It is known that by increasing the angle of incidence on the retreating blade in this manner that an angle of incidence is reached whereby the retreating blade stalls, thus limiting the forward speed of the helicopter.

The application of second harmonic control has often been suggested in the past as a means of decreasing the angle of attack on the retreating blade to values below the stall angle while, at the same time, maintining approximately the same lift on the advancing and retreating blades and, also, maintaining the total lift of the rotor. While the value of second harmonic control has been recognized particularly to achieve high speed helicopters, second harmonic control has never been applied successfully. The principal reason that second harmonic control has not been applied successfully to rotary wing aircraft has been the mechanical complexity required for its introduction. The addition of complex gears, bearings, and the like, to an already intricate pitch control mechanism has been looked upon with such disfavor as to cause its rejection in practice.

A comprehensive treatment of the application of second harmonic control to helicopters has been presented in Journal of the American Helicopter Society, vol. 6, No. 2, April 1961 pp. 8-l9, Peter I. Aricidiacono, Theoretical Performance of Helicopters Having Second and Higher Harmonie Feathering Control. A principal conclusion of this detailed presentation is that a simple second harmonic control system could not be devised with amplitude and phase control adjuncts.

According to the present method, a simple second harmonic control is established with both amplitude and phase control adjuncts. Rotational movement of the helicopter mast is simply transformed into rotor blade pitch change by articulately linking second harmonic feathering control components and a mast-attached sec- 0nd harmonic swashplate upon perpendicular axes. Amplitude control is provided by tilting of the second harmonic swashplate and phase control is provided by azimuthly re-orienting the second harmonic swashplate upon its horizontal axis.

Accordingly, it is an object of this invention to provide a simple method of second harmonic control in rotary wing aircraft.

Another object of invention is to provide a method of second harmonic control for rotary win-g aircraft with complete amplitude and phase control adjuncts.

Another object of invention is to provide a method for 2 simplified harmonic control, as an assistance in obtaining high speed rotary wing flight and in obtaining reduction of vibrations and rotor loads.

1 Further objects of invention will become apparent from the ensuing specification and attached drawings wherein:

FIGS. 1 and 2 are schematic views illustrating the principle of invention wherein links articulated upon perpendicular axes are employed to obtain t-wo reciprocating movements of the feathering control rod 111 per revolution of the mast, FIG. 1 showing the collapsed links 107 and 109 in position of rotation of 0 and 180 and FIG. 2 showing the rigid links 107 and 109 in position of rotation at 9'0 and 270. In these latter two positions second harmonic, pitch change motion is imparted upon pitch control rod 111 and thus the helicopter blades;

FIGS. 3 through 8 show a model apart from any association with a rotor or rotor blades, illustrating the kinematics of the invention, FIG. 3 and FIG. 4 being respectively a front and side elevation of the model structure illustrated in FIGS. 5-8;

FIG. 9 illustrates the application to a two bladed rotor of a second harmonic feathering control which is adjustable in both amplitude and phase, according to the present method.

As shown in FIGS. 3 and 4, non-rotating plate 101 is pivoted along axis X-X to fixed or base structure 103. Rotataole shaft 105 attachable to a helicopter mast is journaled in non-rotating tiltable plate 101 and is pivotally attached to link 107 along axis Y-Y. Link 107 extends upwardly to pivot on link 109 upon axis Z-IZ which is perpendicular to axis Y-Y. Link 109 attaches to feathering control rod 111, which slides in guide 113, upon axis 8-5 which is parallel to axis ZZ and also perpendicular to axis YY. Amplitude control tilting rod 115 is connected to non-rotating plate 101.

Link 109 side facing the reader and as illustrated in FIG. 5, is marked with the letter A, the side to the right of the reader is B, the side away from the reader is C, and the side to the left of the reader is D. Thus with every rotation 'of the mechanism by means of shaft a new letter will present itself to the viewer, a illustrated in FIGS. 5 through 8.

Assume that the elements 105, 107, 109 and 111 are rotating uniformly. FIG. 5 shows 0 position of the mechanism with the control rod 115 actuated so as to tilt plate 101. 90 later in space, the mechanism takes up the position shown in FIG. 6. Link 107 in the position shown in FIG. 6 is forced to tilt perpendicularly to the tilt plane of plate 101, the perpendicular relation between axis Y-Y, on the one hand, and the parallel axes ZZ and S-S, on the other hand, permitting this tilt of link 107, causing rod 111 to descend, as shown. 90 later as illustrated in FIG. 7, the axes ZZ and SS do not permit a tilt of link 107, and axis Y-Y now being perpendicular to the plane of the drawing surface, the mechanism is free to rise as shown. FIG. 8 is effectively a repetition of FIG. 6, although removed fromFIG. 6. Thus, :as seen by the motions of rod 111, a rotary motion is transformed into a twice per revolution reciprocating motion. This two per revolution reciprocating motion may be transmitted to the rotor blades to effect a second harmonic feathering control.

One suggested mechanism showing how the two per revolution reciprocating motion may be transmitted to the rotor blades is illustrated in FIG. 9. It should first be understood that the mechanisms illustrated in (FIGS. 1-8 are upside down with reference to the embodiment of FIG. 9. In FIGS. l-8 the mechanism is illustrated as operating from the bottom upwardly to the blade, Whereas in FIG. 9 the equivalent mechanism is illustrated to operate downwardly to the blade. To clarify, 107 in FIG. 1

is the equivalent to 9 in FIG. 9, 109 (FIG. 1) is the equivalent of 11 (FIG. 9) and 111 (FIG. 1) is the equivalent of 13 (FIG. 9). With reference to FIGS. 3-8 the same equivalencies apply and it can also be seen that 101 (FIGS. 3-8) is the equivalent of '19 (FIG. 9) and 115 (FIGS. 38) is the equivalent of 3 ('FIG. 9), links 115 and 3 respectively furnishing variation and amplitude capabilities. In FIG. 9 sleeve 17 is positioned internally of rotor mast 15 land is held in position along the longitudinal axis of the mast. Inner ring 19 is pivoted on sleeve [17 along axis GC and is pivotally attached to amplitude control link '3. Link 3 extends through and coaxially with sleeve 17 and is used to tilt inner ring 19 with relation to mast 1-5 to introduce second harmonic amplitude feathering control to the rotor.

Outer ring 35 is heldadjacen-t the inner ring 19 through bearings 21. Tubes 23 extend radially from outer ring 35 to attach to ring 9 along axis E E. Ring 9 pivots on mast 115 along axis DD, which axis is perpendicular to axis EE. Axes D-D and EE, and C--C are all essentially in the same plane. The azimuthal orientation of inner ring 19 axis CC determines the phasing of the second harmonic input and will be discussed hereinafter. Link 11 pivots to ring 9 at axis F-'F and to slider 13 at axis GG. Slider 13 is attached to mast 15 through spline 21.

The suggested mechanism imparts a second harmonic reciprocating pitch change motion to slider 13 and from slider 13 to rotor blades 45 and 46 in the following manner. 1f tube 3 is lowered by means of bell crank 97, it will be seen that the second harmonic swashplate 37 consisting of inner ring 119, bearings 21 and outer'ring 35 is tilted around axis C-C. Elf we assume that rotating axis D-D is momentarily aligned with non-rotating axis CC, as illustrated in FIG. 9, then at that time ring 9 will be parallel to the second harmonic swashplate 37 thus causing axis F-fl? to move laterally which will cause slider '13 to move upwardly along the spline 21. Now consider that the mast 15 rotates 90 which would place rotating axis EE in line with non-rotating axis C-C. The articulate connections of ring 9 to link 11 and link 11 to slider 13, through axes F l and GG require the ring 9 to be perpendicular to the axis of mast 15 when EE is aligned with C-C. This causes the link 11 to be aligned parallelly to the mast and causes slider 13 to move downwardly along spline 21. The following 90 rotation of the mast would cause slider 13 to rise to its original position, slider 13 thus completing one up and down cycle every 180 of mast rotation, or 2 cycles every 360 of mast rotation. it now remains to transmit this two per revolution reciprocating motion to rotor blades 45 and 46 which may be accomplished according to the FIG. 9 construction as follows:

Lever 25 is pivoted at one end to slider 13 and at the other end to cyclic pitch control tube 27 which extends from a conventional cyclic control main swashplate (not shown). Link 29 is attached at one end to link 25 and at the other end to pitch horn 31 which extends from blade grip 33 that bolts on to rotor blade 45 and 46 at point 39. As slider 13 moves up and down twice per revolution, link 25 is caused to pivot around point 41 imparting a 2 per revolution second harmonic pitch change movement to pitch horn 31. This motion is superimposed on the blade motion cyclic pitch control introduced by control rod 27 extending up from the main swashplate.

As noted previously, application of second harmonic feathering control to the blade for the purpose of delaying the blade stall will require that a maximum decrease of blade pitch be more or less in the azimuths transverse to flight. Considering the direction of flight as forward and the rearward azimuth of the rotor designated as with counterclockwise rotation of the rotation of the rotor in top plan view, then the advancing blade is at 90 and the retreating blade is at 270 and it is substantially in these suppressing vibration and/ or blade loads, it may be desirable to introduce second harmonic feathering control in quite a different phase relationship than that for the purpose of delaying retreating blade stall. Thus, it may be desirable to have the maximum decrease of blade pitch introduced in azimuths substantially different from or 270 and it may even be desirable to eifect some compromise of second harmonic phasing other than that optimumly required for the delay of retreating blade stall or that required for suppression of rotor blade vibrations. Thus, it would be extremely beneficial for the pilot to be able to conveniently shift the phasing of the second harmonic control and be able to select the best phasing for the particular mission and the particular situation.

Similarly, it can be very advantageous to be able to easily adjust the amplitude of the second harmonic input to the rotor. For example, during hovering the application of second harmonic control could introduce vibrations and/ or blade loading forces. The second harmonic feathering control could be cancelling out forces in a condition of forward flight while introducing forces in a condition of hovering. This would indicate the need for a correlation or interconnection between the cyclic stick and the amplitude of the second harmonic control input, and it would be desirable for the pilot to be able to control the amplitude of the second harmonic feathering input to the rotor. In both cases, that of the phasing and that of the amplitude of the second harmonic control, the adjustment can be manually directed by the pilot or it can be connected to an appropriate flight control for automatic' adjustment.

in FIG. 9 the azimuthal position of axis C-C determines the phasing of the second harmonic feathering control. For example, in FIG. 9, let us consider that the direction of forward flight is toward blade 45 away from .the reader and that the main swashplate (not shown) tilts conventionally down in the forward position for forward flight. This would mean that when the blades are rotated 90 from the position shown in FIG. 9 retreating blade 45 to the left would have an increase in pitch and advancing blade 46 to the right would have a decrease in pitch as a result of the main swashplate. In the position shown in FIG. 9, blades 45 and 46 being aligned with the direction of flight, the pitch on both blades is substantially equal. As heretofore noted, an object of second harmonic control is to delay the stall on the retreating blade by decreasing the pitch angle on the retreating blade and decreasing the pitch on the advancing blade, while the total lift of the rotor is maintained constant by increasing the pitch of the blades when they are in the longitudinal or fore and aft position. Swashplate 3-7 tilted upon the axis C-C as illustrated in FIG. 9 will cause the blades in the longitudinal position to increase pitch. 90 later with the blades in the lateral azimuth slider :18 will descend to its lower position and the pitch in the blades (advancing and retreating) will be decreased. If it is advantageous to shift the phasing of the second harmonic control, that is to have the maximum decrease of pitch take place in an azimuth other than a lateral azimuth, then it is a simple matter according to the present method horizontally to rotate tube 17, as desired and by means of azimuth control rod shown pivotally attached to tube flange 94 at point 93. Azimuth control rod 95 could be attached to a control stick manipulated by the pilot or could be attached to an existing flight control lever. Horizontal rotation or re-orientation of axis C-C by these means would thus effect an azimuthal shift in the phasing of the second harmonic feathering control.

The amplitude of the second harmonic feathering control can be adjusted by the degree of tilt of the second harmonic swashplate 37 upon axis C- C and is regulable by amplitude control tube 3 shown connected to bell crank 97 which is actuated by control tube 99, which can also either be attached to a specific lever under the control of the pilot or can be attached to an appropriate existing flight control such as the cyclic stick.

Manifestly, generous modifications in the single illustrated structure may be employed without departing from the spirit and scope of invention, as defined in the subioined claims.

I claim:

1. In the driving of a helicopter of the type having a rotatable mast with rotor blades adjustably fixed thereto and including a first harmonic cyclic pitch control, a method for transforming rotational movement of said mast into a rotor blade second harmonic pitch control, comprising:

(A) interposing between said mast and said first harmonic cyclic pitch control a first link articulated upon a. first axis in a tiltable plane and a second link connected to said first link upon a second axis perpendicular to said first axis, said second link being connected to said first harmonic cyclic pitch control;

(B) rotationally driving said second link from said mast; and

(C) tilting the axis of said first link to provide reciprocating movement amplitude control.

2. Method as in claim 1, including:

(D) azimuthly re-orienting said first axis to provide reciprocating movement phase control.

3. A second harmonic control system on a rotary wing aircraft having rotor blades supported upon a mast and pitch-changeable by means of a firstharmonic cyclic pitch control system, comprising:

(A) a second harmonic swashplate pivotally mounted on said mast;

(B) a first link pivotally attached at one end to said second harmonic swashplate along a first axis lying in a plane parallel to the plane containing the axis of pivotal attachment between said swashplate and said mast;

(C) a second link pivotally attached to the other end of said first link along a second axis perpendicular to said first axis; and

(D) a second harmonic cyclic pitch lever interconnecting said second link and said first harmonic cyclic control system.

4. A second harmonic control on a helicopter comprising:

(A) a mast;

(B)pitch-changeab1e rotor blades attached to said mast;

(C) first harmonic cyclic pitch means connected to said blades;

(D) a second harmonic swashplate having a rotatable portion and non-rotatable portion, said non-rotatable portion being pivoted to said mast;

(E) a first link pivotally attached at one end and upon a first axis to the rotatable portion of said second harmonic swashplate;

('F) a second link pivotally attached to the other end of said first link on second axis perpendicular to said first axis; and

(G) a second harmonic cyclic pitch control lever pivotally attached to said second link on an axis parallel to said second axis and connected to said first harmonic cyclic pitch lever.

5. A second harmonic control on a helicopter comprising:

(A) a mast;

(B) rotatable, pitch-changeable blades attached to said mast;

(C) first harmonic cyclic pitch means contacting said mast and said blades; and

(D) second harmonic cyclic pitch means interconnecting said first harmonic cyclic pitch means and said mast, further including:

1) a second harmonic swashplate having a rotatable portion and a non-rotatable portion, said non-rotatable portion being pivoted to said mast;

(2) a first link pivotally attached to said swashplate rotatable portion upon a first axis;

(3) a second link pivotally attached to said first link upon a second axis perpendicular to said first axis, and

(4) a slider supported upon said mast and pivotally connecting said second link and said first harmonic cyclic pitch means via a second harmonic cyclic pitch control lever whereby when said blades are rotated around the axis of said mast, and when said second harmonic swashplate is tilted on its pivoted mountings said slider is oscillated lineally at a frequency twice that of the rotational frequency of the blades, imparting a second harmonic pitch change to said blades.

6. A second harmonic control as in claim 5, including means limiting the degree of tilt of said swashplate as a second harmonic amplitude control.

7. A second harmonic control as in claim 6, said second harmonic swashplate being azimuthly repositionable as a measure of second harmonic phase control.

References Cited in the file of this patent UNITED STATES PATENTS 2,410,545 Main Nov. 5, 1946 2,983,319 Kaman May 9, 1961 3,031,017 Arcidiacono Apr. 24, 1962 OTHER REFERENCES Journal of the American Helicopter Society, Apr. 1961, pg. 19, vol. 6, No. 2. 

1. IN THE DRIVING OF A HELICOPTER OF THE TYPE HAVING A ROTATABLE MAST WITH ROTOR BLADES ADJUSTABLY FIXED THERETO AND INCLUDING A FIRST HARMONIC CYCLIC PITCH CONTROL, A METHOD FOR TRANSFORMING ROTATIONAL MOVEMENT OF SAID MAST INTO A ROTOR BLADE SECOND HARMONIC PITCH CONTROL, COMPRISING: (A) INTERPOSING BETWEEN SAID MAST AND SAID FIRST HARMONIC CYCLIC PITCH CONTROL A FIRST LINK ARTICULATED UPON A FIRST AXIS IN A TILTABLE PLANE AND A SECOND LINK CONNECTED TO SAID FIRST LINK UPON A SECOND AXIS PERPENDICULAR TO SAID FIRST AXIS, SAID SECOND LINK BEING CONNECTED TO SAID FIRST HARMONIC CYCLIC PITCH CONTROL; (B) ROTATIONALLY DRIVING SAID SECOND LINK FROM SAID MAST; AND (C) TILTING THE AXIS OF SAID FIRST LINK TO PROVIDE RECIPROCATING MOVEMENT AMPLITUDE CONTROL. 